Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

Gold nanorods: Synthesis, characterization and applications

We use the discrete dipole approximation to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local electric field (E-field) enhancement near the particle surfaces. The results are used to determine what conditions are most favorable for producing enhancements large enough to observe single molecule surface enhanced Raman spectroscopy. Most of the calculations refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addition, for the dimer calculations we study the influence of dimer separation and orientation, especially for dimers that are separated by a few nanometers. We find that the largest /E/2 values for dimers are about a factor of 10 larger than those for all the monomers examined. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given /E/2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest /E/2 values we have calculated for spacings of 2 nm or more is approximately 10(5).

Electromagnetic fields around silver nanoparticles and dimers.

Noble metal nanoparticles are capable of confining resonant photons in such a manner as to induce coherent surface plasmon oscillation of their conduction band electrons, a phenomenon leading to two important properties. Firstly, the confinement of the photon to the nanoparticle's dimensions leads to a large increase in its electromagnetic field and consequently great enhancement of all the nanoparticle's radiative properties, such as absorption and scattering. Moreover, by confining the photon's wavelength to the nanoparticle's small dimensions, there exists enhanced imaging resolving powers, which extend well below the diffraction limit, a property of considerable importance in potential device applications. Secondly, the strongly absorbed light by the nanoparticles is followed by a rapid dephasing of the coherent electron motion in tandem with an equally rapid energy transfer to the lattice, a process integral to the technologically relevant photothermal properties of plasmonic nanoparticles. Of all the possible nanoparticle shapes, gold nanorods are especially intriguing as they offer strong plasmonic fields while exhibiting excellent tunability and biocompatibility. We begin this review of gold nanorods by summarizing their radiative and nonradiative properties. Their various synthetic methods are then outlined with an emphasis on the seed-mediated chemical growth. In particular, we describe nanorod spontaneous self-assembly, chemically driven assembly, and polymer-based alignment. The final section details current studies aimed at applications in the biological and biomedical fields.

Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications

Gold nanorods have been synthesized by photochemically reducing gold ions within a micellar solution. The aspect ratio of the rods can be controlled with the addition of silver ions. This process reported here is highly promising for producing uniform nanorods, and more importantly it will be useful in resolving the growth mechanism of anisotropic metal nanoparticles due to its simplicity and the relatively slow growth rate of the nanorods.

Photochemical synthesis of gold nanorods.

Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic Au(core)M(shell) (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.

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Crystal Overgrowth on Gold Nanorods: Tuning the Shape, Facet, Aspect Ratio, and Composition of the Nanorods

Recent research in the field of nanometer-scale electronics has focused on two fundamental issues: the operating principles of small-scale devices1 and schemes that lead to their realization and eventual integration into useful circuits.2 The availability of a nanoscale toolbox is the key for this field of research. Among the many potential building blocks within this nanoscale toolbox, nanowires are considered one of the key components because they can be used as interconnects and other functional devices in nanoelectronics.3 Unfortunately, although several processes have been developed for the syntheses of semiconductor nanowires,4 few methods have been developed for preparing free-standing, uniform metal nanowires. Among them, template synthesis (in porous matrixes such as porous Al2O3 films5 and mesoporous silica6) and step-edge decoration7 are considered as effective approaches. The step-edge decoration method was recently employed to produce Mo nanowires of 100 nm thickness.7 Metal nanowires have also been prepared by using DNA8 and carbon nanotubes as templates.9 Herein we report a simple chemical process for synthesizing long, free-standing metal nanowires. LiMo3Se3 nanowires are used as both reducing agents and sacrificial templates in this study to yield continuous metal nanowires. The metal nanowires generally have diameters of 10100 nm and lengths of several micrometers. These metal nanowires display small ohmic resistances at room temperature, indicating that these wires could prove useful as interconnects in nanoelectronic circuits. Previously, DNA has been used as a biotemplate for making metallic nanowires,8 although the continuity of the resulting metal nanowires has been problematic. Since DNA itself does not possess any reducing/oxidizing capability, the synthesis generally is a two-step process, which involves metal activation followed by chemical reduction. Nevertheless, the construction of electronic circuits based only on native DNA remains problematic, mainly due to the high resistance of DNA that limits its potential applications in this regard. Recently, Braun and co-workers presented a new approach by fixing DNA between two contacts and utilizing it as a template for the construction of a silver nanowire.8 This technique uses the molecular recognition properties of the molecule for the defined buildup of a nanostructure and installs its electrical functionality by the directed construction of a metallic wire on the biotemplate. However, the reported 100 nm thick silver wires displayed a nonconducting gap for small bias voltages. In this study, we choose [Mo3Se3]∞ molecular chains as our experimental system for two important reasons. First, these molecular chains are obtained by dissolution of crystals of quasi1D materials LiMo3Se3 in polar solvent.10 Each chain is made of staggered stacks of triangular Mo3Se3 with a diameter of 0.5 nm. The molecular chain itself is a reducing agent and can be readily oxidized. Second, these individual molecular wires form uniform nanowire bundles with diameters of 2-100 nm in certain polar solvents such as methanol and DMSO. Hence, these nanowires possess dual functionalities, being reductive one-dimensional templates. In fact, the redox chemistry of LiMo3Se3 was previously studied by Tarascon, and it was demonstrated that insertion and extraction of Li/Mo6Se6 is reversible.11 To form uniform metal nanowires, a redox reaction is carried out using LiMo3Se3 nanowires as the reducing agents to reduce aqueous metal ions (e.g., AuCl4, Ag+, PdCl4, PtCl4). Metal ions are reduced and deposited directly on the nanowire templates, while the nanowire templates are oxidized into Mo3Se3 and eventually dissolve in water (Figure 1). Consequently, metal nanowires with diameters of 2-100 nm can be obtained through this cooperative chemical templating process. Metal nanowires of Au, Ag, Pt, and Pd can be readily synthesized. In a typical experiment, a LiMo3Se3 solution was prepared by dissolving 5 mg of LiMo3Se3 in 20 mL of DMSO. A 2 mL drop of metal ion solution (0.005 wt %) was added to this DMSO solution. Using the Au nanowire system as an example, evidence of the reaction is apparent immediately after the solution mixing. The overall solution color develops a pink tint, and a peak at 526 nm appears in the UV-vis absorption spectra, indicating that Au nanocluster nucleation and growth has occurred. Figure 2 shows transmission electron microscopy images taken before (a) and after (b-d) the templating reaction for the Au nanowire system. Before the reaction, the LiMo3Se3 molecular chains self-assemble into nanowire bundles of 10-100 nm in diameter. After the redox reaction, Au nanowires of similar diameters and morphology are obtained. These Au nanowires are continuous and polycrystalline. Figure 2d shows a high-resolution TEM image of an individual Au nanowire with diameter of 15 nm. It can be seen that the Au nanocrystalline domains fuse together at the interface and form a continuous and robust (1) Dekker: C. Phys. Today 1999, 52, 22. (2) (a) Hu, T.; Odom, W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (b) Collier, C. P.; Wong, E. W.; Behloradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (c) Rueches, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C.; Lieber, C. M. Science 2000, 289, 94. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (4) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (5) (a) Foss, C. A.; Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1992, 96, 9001. (b) Preston, C. K.; Moskovits, M. J. Phys. Chem. 1993, 97, 8495. (c) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316. (6) Huang, M.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 12, 1603. (7) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120 and references therein. (8) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (9) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (10) Tarascon, J. M.; DiSalvo, F. J.; Carrol, C. H. J.; Walsh, M.; Rupp, L. J. Solid State Chem. 1985, 58, 290. (11) Tarascon, J. M.; Hull, G. W.; DiSalvo, F. J. Mater. Res. Bull. 1984, 19, 915. Figure 1. Schematic illustration of metal nanowire templating reaction between LiMo3Se3 nanowires and metal ions. 10397 J. Am. Chem. Soc. 2001, 123, 10397-10398

Metal Nanowire Formation Using Mo3Se3- as Reducing and Sacrificing Templates

Metal Nanowire Formation Using Mo3Se‐3 as Reducing and Sacrificing Templates.

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Jae Hee Song

Papers

Photochemical synthesis of gold nanorods.

Crystal Overgrowth on Gold Nanorods: Tuning the Shape, Facet, Aspect Ratio, and Composition of the Nanorods

Metal Nanowire Formation Using Mo3Se3- as Reducing and Sacrificing Templates

Metal Nanowire Formation Using Mo3Se‐3 as Reducing and Sacrificing Templates.

Photochemical Synthesis of Gold Nanorods.